A communication system such as a mobile phone or portable telephone utilizes a mobile terminal or portable terminal that is compact and lightweight and is suitable to be carried around, and fixed base station communication equipment for communication with each of the portable terminals. Most of radio waves used in most of the communication systems are classified into a frequency band for base station reception that is oscillated at and transmitted from the portable terminal and received at the base station communication equipment, and a frequency band for base station transmission that is oscillated at and transmitted from the base station communication equipment and received at the portable terminal. In either the portable terminal and the base station communication equipment, such devices separates radio waves so that a transmission signal and a reception signal do not interfere with each other by using a filter, and carries out signal processing with respect to each of the radio waves.
As the filter used in the portable terminal, there is a ceramic filter, as described in Patent Document 1, for example. Weight reduction and size reduction have been advanced for the portable terminal along with the advance of technology so as to be easily carried around, and the filter used therein has also been reduced in size.
On the other hand, in a base station side, due to a wide communication area, radio waves need to be radiated from an antenna by using high power communication equipment. For this reason, as a transmission side filter for the base station communication equipment, one with a low loss has been required in order to supply a high powers amplified by a semiconductor device and so on, to the antenna economically. In particular, in the transmission side filter, a configuration needs to be such that the temperature of the filter itself does not increase as a result of the generation of heat due to a loss by the filter.
For this reason, in the base station communication equipment, as described in Non-Patent Document 1 for example, a filter of a type that uses, as a resonant element, a metallic rod inserted in a metallic enclosure, which is so-called a reentrant coaxial or cylindrical cavity resonator. Such a filter has a low loss, and has good heat release and conductivity in order for the generated heat to be less likely to cause a temperature increase since the filter is composed of metal. However, the reentrant coaxial or cylindrical cavity resonator has a large size and is expensive.
In recent years, a communication system of high speed and large capacity has been achieved, and a communication system using 1.5 to 4 GHz as a communication frequency band has been provided. In such a communication system, in accordance with increases in the number of the portable terminals and in lines of the base station, there is a tendency that a wide area is covered by narrowing a communication area of one cell, and instead thereof, increasing the number of the cells. For this reason, the number of the base stations has also been increased. If one including the reentrant coaxial or cylindrical cavity resonator as described above is used for the filter of the communication equipment installed in a number of the base stations as described above, problems that the filter is large, heavy, and expensive and not suitable for mass production become apparent.
In view of the above problems, there has been a desire for a practical filter for the base station communication equipment that is compact and lightweight, and inexpensive and can be mass-produced easily, in place of the filter including the reentrant coaxial or cylindrical cavity resonator.
From the viewpoint of size reduction and weight reduction, it is preferable that dielectric ceramics having a high relative dielectric constant be used. That is, a dielectric resonator has a length in an axial direction substantially proportional to 1/(relative dielectric constant)1/2, and the higher the relative dielectric constant becomes, the more a dimension in the axial direction becomes shortened. The reentrant coaxial or cylindrical cavity resonator is a ¼ wavelength dielectric resonator using air (having a relative dielectric constant of around 1) as a dielectric material, and a length in the axial direction of the reentrant coaxial or cylindrical cavity resonator having a resonance frequency at 2 GHz is 37.5 mm. On the other hand, a ¼ wavelength dielectric filter using dielectric ceramics having a relative dielectric constant 40 has a length in the axial direction of the resonator of 5.93 mm, and is shortened to around ⅙ of the length of the reentrant coaxial or cylindrical resonator.
As a filter for the base station communication equipment formed by using a block of dielectric ceramics for use in a communication system of a frequency band lower than 1 GHz, as described in Patent Document 2, for example. In a specific example shown there, a block of ceramics having a relative dielectric constant of 40 is used, and dimensions of the block is 77.6 mm in length, 11.74 in width, and 11.54 in height. A transmission frequency is 825 MHz through 845 MHz and a reception frequency is 870 MHz through 890 MHz, and as an electric characteristic, it is described that an insertion loss is 1.6 dB and a reception signal is attenuated for 55 dB. In addition, also in Patent Document 3, a dielectric filter using a block of ceramics having a relative dielectric constant of 40 is disclosed. Dimensions of the block is 81.3 mm in length, 30.0 mm in width, and 20.4 mm in height, and a transmission frequency is 453 MHz through 457.475 MHz and a reception frequency is 463 MHz through 467.475 MHz.
However, a practical dielectric filter for base station communication equipment that uses dielectric ceramics and is used in a frequency band equal to or higher than 1 GHz has not conventionally proposed.
Note that Patent Document 4 discloses dielectric ceramics having a high dielectric constant, a large Q, and a good temperature dependency characteristic of a resonance frequency as a material of a dielectric resonator used at a frequency band equal to or higher than 1 GHz. However, at a frequency band equal to or higher than 1 GHz, when a dielectric block composed of dielectric ceramics having a high dielectric constant is used, dimensional accuracy is strictly required for the dielectric block, and difficulty in processing increases, manufacturing cost increases, and it becomes impractical. In view of the above, as described in Patent Document 5 for example, it is suggested that dielectric ceramics having a relative dielectric constant of 10 to 20 is used to lower a dielectric loss, and also lower a temperature factor or coefficient of a resonance frequency. In addition, Patent Document 6 discloses dielectric ceramics having a relative dielectric constant equal to or lower than 20, a low temperature factor or coefficient of a resonance frequency, and a high Q value.    Patent Document 1: WO-A1-2000-515336    Patent Document 2: WO-A1-59-500198    Patent Document 3: JP-A-61-262301    Patent Document 4: JP-A-04-300243    Patent Document 5: JP-A-2002-201062    Patent Document 6: JP-A-2002-80273    Non-Patent Document 1: MICROWAVE FILTERS, IMPEDANCE-MATCHING NETWORKS, AND COUPLING STRUCTURES SEC. 8.13, COMB-LINE, BAND PASS FILTERS, McGraw-Hill 1964